This project is a study of the relationship of the hormone, calcitonin, to plasma phosphate concentrations and the movement of phosphate into and out of bone and soft tissues. It has four primary objectives: (1) To study the physiological effects of calcitonin on phosphate movement. This will involve a determination of the effect of calcitonin on phosphate absorption from the gut, its removal from plasma, and the site of its deposition in hard and soft tissue. (2) To study the relationship of the effect of calcium and of calcitonin on plasma phosphate concentration. Since calcium raises plasma phosphate levels, and calcitonin lowers these, this portion of the project will be aimed at determining if a primary physiological action of the hormone may not be to prevent the adverse effects of high calcium intake on phosphate movement into or out of blood. (3) To study the relationship of the hypophosphatemic effect of calcitonin to the hypocalcemic effect of the hormone. This portion of the project is to test the thesis that the hypocalcemic effect of calcitonin may be the result of a prior effect on phosphate movement in bone tissue. (4) To search for a biochemical basis for calcitonin's influence on phosphate movement. This is to test the thesis that calcitonin acts by increasing phosphate movement across cell membranes, and thereby may require enzymatic aid for active transport.